UPDATE: In-depth instructions now available at: https://www.instructables.com/id/RFID-NFC-Tap-and-Go-Ring-for-Credit-Card-Payment/
Video of me using it at: https://youtu.be/5hDjPP-w8Bc
I have included AutoDesk Inventor 2020 files. Note that I'm still learning 3D CAD and I make no promises as to how neat or well-done these files are. If you'd like me to export them in another format, I'd be happy to do so. Please DM me if you need them in another format.
These files are for a 10 and 3/4 sized ring and inner frame, the idea being that if you have an RFID credit card, you can tell your bank that you need a replacement card, once your bank sends you a second card, you can dissolve it in acetone, (I found 1 - 2 hours if you scraped off the back and front coating of the card worked well, although more doesn't seem to hurt the RFID chip anyways. Keep an eye on the progress and take it out when it looks clean.) take out the RFID chip, solder a new antenna to it, glue that assembly to the inner frame STL, then put that assembly in a mold of the full-size ring, pour resin around it, then have a ring that will let you pay at any establishment that accepts RFID credit cards (pretty much anywhere that accepts google/apple pay) without you needing to take out your wallet.
(Obviously don't tell your bank that your card was stolen. Then they won't give you a copy, instead they'll change the card number. For most banks, you just need to choose the "my card is worn or damaged" option.)
To find out if your card has RFID, look for this symbol on the front or back: https://imgur.com/rQDpAac
If it doesn't, you can also call your bank and see if they have them available. My card did not have RFID but they were able to issue me one that did when I called and asked.
If your ring size isn't 10 and 3/4, scale it using this chart before you print it.
Since the inside diameter is shown as 0.806 inches (You're looking at the radius in the last picture above, so we're doubling it.) If your ring size is 9 and 1/2, looking at the chart you'll see that's a diameter of 0.762 inches. 0.806 divided by 0.726 is 0.9454ish, so just scale the models to 94.5% of their current size before you print them. Note that bigger ring sizes mean more room for more resin around the RFID chip, so I wouldn't suggest making this a pinky ring.
You'll probably want to do the inner frame (and realistically the ring itself too) on a resin printer. An FDM printer won't be able to make the little arms. I used a form 2 and it worked great.